1. Field of the Invention
A process for making or repairing ion exchange membranes from a solution of fluorinated polymers is disclosed. The solution comprises perfluoro cycloalkane, perfluorinated aromatic compounds or perfluorotrialkyl amine solvents and fluorinated polymers having carboxyl groups and an equivalent weight greater than 900. The solvents have a critical temperature greater than 150.degree. C.
2. Technical Background
The present invention relates to electrolytic cells and particularly to electrolytic halogen generating cells in which a ion exchange membrane separates the anode and cathode within the electrolytic cell. Specifically, this invention relates to methods for making and for repairing these ion exchange membranes. More specifically, this invention concerns a process for making ion exchange membranes from solutions of fluorinated polymers having carboxyl groups and an equivalent weight greater than 900. The solutions are made from novel solvents which dissolve certain alkyl carboxylate forms of fluorinated polymers which are copolymers of tetrafluoroethylene ("TFE") and certain perfluorovinylether comonomers. The solvents readily dissolve such polymers more completely and at lower temperatures than previously disclosed solvents, an advantage in equipment utilization and other manufacturing concerns. The ion exchange membranes are particularly useful in an electrolytic cell such as a chloralkali cell or a fuel cell. The solutions are also useful as corrosion resistant spray coatings for a multitude of materials.
The prior art generally discloses the solubility of certain fluorinated polymers but there is no mention of full solutions of carboxylic fluorinated polymers having high equivalent weights in this class of inventive solvents, particularly at low temperatures. There is also no mention of a process to make or repair ion exchange membranes from such solutions.
Generally, ion exchange membranes are between 0.5 and 150 mil in thickness. Being thin, these membranes, while strongly resistant to the chemical environment within the electrolytic cell or fuel cell, are often subject to physical damage: tears, punctures and flex fatigue cracking. One past proposal has been to repair this physical damage using low equivalent weight copolymer solvated with an alcohol. It is difficult to fully dissolve the polymers in an alcohol solvent. In addition, the repaired areas have not offered desirable membrane performance characteristics normally associated with higher density copolymeric material, resulting in decline of the overall performance of the membrane. These repairs often have achieved less than desirable adhesion to the membrane because mechanical bonding not solvent molding is a significant factor in adhesion.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of perfluorocarbon composite electrodes and for solvent welding the composites to perfluorocarbon membranes where the perfluorocarbons are of relatively elevated equivalent weights desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for fully solvating these higher equivalent weight perfluorocarbons.
Some solvents are known in the prior art for this class of carboxylic fluorinated polymers; however, some of the solvents are very expensive or require high temperature to dissolve the polymer. The prior art teaches solvents for carboxylic fluorinated polymers with equivalent weights below about 900 and solvents for such polymers which require elevated temperatures or pressures. There is no teaching of a solvent for the ester form of carboxylic fluorinated polymers with an equivalent weight greater than 900 at low temperatures.
For example, U.S. Pat. Nos. 4,650,551; 4,778,723 and 4,784,900 disclose solutions near room temperature of highly swollen dispersions of resins which are copolymers of TFE and perfluorovinylether comonomers with acidic end groups having low equivalent weights less than 850. Heavily fluorinated alkanes, like 1,2-dibromotetrafluoroethane and 1,1,2-trichlorotrifluoroethane were used as "solvents". Examples of room temperature solutions were also given using polyhalogenated alkyl ethers with boiling points less than 190.degree. C.; the equivalent weight for these resins was exceedingly low at 690 or lower.
Fluorinated polymer resins commercially known as NAFION.RTM. (available from E. I. du Pont de Nemours and Company) with an equivalent weight greater than about 1000, must be dissolved at elevated temperatures and pressures due to the presence of crystallinity. U.S. Pat. Nos. 4,266,036; 4,272,560 and 4,298,697 discuss the use of high boiling perhalogenated alkyl ethers and perhalogenated alkanes as solvents at elevated temperatures. The examples describe the dissolution of 1050 to 1200 equivalent weight resins in oligomers of chlorotrifluoroethylene ("CTFE") at temperatures of about 225.degree. to 250.degree. C.; however, the polymers are more likely to decarboxylate at high temperatures. Chemical degradation of the polymer may occur in solutions of CTFE oligomer at 200.degree. C.
U.S. Pat. No. 4,535,112 describes a method of repairing ion exchange membranes using dispersions or partial solutions of 1050 to 1200 equivalent weight polymers in a wide variety of heavily fluorinated liquids for purposes of repairing cation exchange membranes. These liquids include perfluorodecanoic acid, perfluorotributylamine, pentafluorophenol, pentafluorobenzoic acid, perfluoro-1-methyldecalin and decafluorobiphenyl. There is no mention of full solutions of carboxylic fluorinated polymers having high equivalent weights at low temperatures.
An article entitled "Solubility Characteristics of Perfluorinated Polymers with Sulfonyl Fluoride Functionality" by McCain and Covitch discusses the solubility of so-called sulfonyl fluorinated polymers, but does not mention alkyl carboxylate forms of fluorinated polymers.